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The Yeast Plasmid 2μ Circle

James R. Broach


Investigation of the yeast plasmid 2μ circle has been pursued in a number of laboratories as a result of the recognition of its potential both for the study of eukaryotic DNA replication and as a cloning vector for yeast. In addition, its ability to interconvert between two separate structures by intramolecular recombination has fostered speculation that expression of the molecule may be modulated by DNA rearrangement. Thus, 2μ circle can also serve as a model system for specialized recombination in a eukaryotic cell with the possibility that this system will provide insights into an unusual form of gene regulation.

In this article I plan to review current knowledge on the cellular location, structure, expression, and functions of 2μ circle with an emphasis on those results that concern the above three topics. By necessity, certain aspects of these topics are covered in other chapters in this volume. The similarities of 2μ circle and chromosomal DNA replication—specifically the dependence of 2μ circle DNA replication on the cell cycle and on chromosomally encoded replication functions—are reviewed by Fangman and Zakian (this volume) and, as a consequence, will not be recapitulated here. However, I will discuss those aspects of 2μ circle replication that distinguish it from chromosomal DNA replication. Similarly, although the use of 2μ circle as a cloning vector is described by Botstein and Davis (1982), the distinctive characteristics of 2μ-circle-mediated transformation, as well as the use of yeast transformation as a means of analyzing 2μ circle functions, is covered...

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